Telecommunication cables, of the type whereby information is transmitted by conduction of electrical signals, have a cable core comprised of a plurality of twisted wire pairs or quads often hereinafter referred to only as pairs. The core is usually protected by a metallic sheath or shield formed about the core and overlaid with an outer, plastic jacket. Where the cable core contains more than some 10 to 25 individually twisted wire pairs it ordinarily assumes a multi-unit configuration. This is necessitated by structural considerations since a cable core formed of a large number of twisted wire pairs merely massed together is too rigid for cable handling and laying facility. Wire pair identification is also inhibited where a large number of wire pairs are formed into a single unit.
The cores of multi-unit type telecommunication cables are thus formed of several individual, distinct units each comprised of a limited number of twisted wire pairs. Each unit is commonly formed by advancing a group of twisted wire pairs serially through an oscillating faceplate, a sizing die and a binder. The faceplate serves to twist the groups of twisted wire pairs together with a group lay that periodically reverses direction. The sizing die and binder respectively serve to compress the group together and to bind them with a helically wound ribbon or tape. Once the units have themselves been formed they are stranded together and bound with a core wrap to form a finished cable core.
The just described multi-unit type of cable has well served the telecommunications industry now for many years. There has however been one recognized, potentially adverse electrical property associated with their use. However, since it has been outside the audio frequency range, the only range actually carried by the cables until recently, its presence has heretofore been one of academic interest only.
Recently, transmission of information has begun to be carried out at higher frequencies as in the transmission of digital data. As a result, what had been a matter of mere academic interest has now become a real, functional problem. The actual problem has to do with insertion loss. Electrical signals transmitted over conductive lines inherently attenuate with distance. For this reason repeaters or regenerators are provided, spaced periodically throughout long routes, to amplify or regenerate the attenuated signal. The magnitude of this attenuation is dependent upon the frequency of the signal transmitted. At lower frequencies the attenuation, or insertion loss as it is termed in the industry, is less than that experienced at higher frequencies. In other words insertion loss is a function of frequency.
When signal strength is plotted against signal frequency a rather smooth curve appears. This, of course, is fortunate since it enables communication system designers to amplify the signals at magnitudes functionally related to the frequency of the signal transmitted. In this manner the strength of the signal received at a reception station, repeater, or regenerator may be equalized, i.e. maintained substantially proportionate to that transmitted over the entire frequency range.
With multi-unit cables the specific problem just discussed rests in the presence of a discrete discontinuity or "notch" in their insertion loss curves. This is exemplified in FIG. 1 wherein a distinct notch 4 is seen to appear in an otherwise smooth insertion loss curve 5 for a multi-unit cable. Since it would be extremely difficult to compensate for this notch with electronic equalizers it becomes necessary to try to prevent its very creation.
Causes of the insertion loss curve notch have been recognized. In multi-unit type cables one such cause is generally attributable to the axial regularity of the appearance of specific wire pairs or quads adjacent to the cable sheath. In other words, the periodicity of the perigee of specific wire pairs in the twisted group forming a unit to the core sheath is regular. This may be better understood by reference to FIGS. 5A-C which schematically illustrate the position of specific wire pairs A through L of a single unit 30 at three axially spaced locations along a cable beneath a tubular metallic cable sheath 31. If in forming the unit the faceplate regularly oscillates through say 340.degree. the A pair will relocate clockwise as illustrated by these sequences until it reaches that location occupied by pair F in FIG. 5A. Then it will reverse and start to occupy other positions on down the line until it reaches that location occupied by pair H in FIG. 5A where it again reverses. Due to the sinusoidal motion of the faceplate oscillation there is a dwell time at each reversal. This causes some wire pairs at twist reversal points to maintain their proximity to the sheath for an extraordinarily long distance. The occurrence of this pair geometry at regular intervals creates impedance changes in these pairs as their electrical field penetrates the metallic sheath. This impedance change causes some of the electrical signal to be reflected back along the cable to provide the insertion loss notch previously mentioned.
With reference next to FIGS. 1 and 2 of the drawing the effect of the just described impedance change may be visualized in a somewhat simplified manner. FIG. 2A illustrates a digital pulse train that includes two pulses 2 and 3 which are sufficiently close in time as to fall within the relatively high frequency range of the discontinuity 4 in the insertion loss curve 5. Insomuch as these two bits of data are transmitted at the frequency within the discontinuity 4 these pulses are attenuated to such a degree as to be below the threshold of the signal receiver. This is identified as ERROR in FIG. 2B wherein the amplitude of the signal received is below the threshold level 6. Conversely, the signals derived from the pulses 7 and 8, which are at a lower frequency, are of sufficient magnitude to be beyond threshold. As a result the pulses 7, 8 and 2 received by the receiver are above threshold and thus properly recognized while the pulse 3 is erroneously not recognized.
Efforts have heretofore been made to solve the just described problem through cable redesign. One such prior art approach has been that of having the faceplates, through which the twisted wire pairs are passed to effect unit twisting, oscillate at substantially faster rates. When this is done the interval between twist reversal points becomes shorter which serves to move the discontinuity or notch in the insertion loss curve to a higher frequency level. However, new digital systems often have higher transmission rates which serve to "chase" the notch shift so achieved. Furthermore, higher frequency transmission results in an increase in the number of impedance changes which cumulatively serve to aggravate or deepen the notch. In other words, since the reflection or impedance change caused by the periodic presence of wire pairs is cumulative, the depth of the discontinuity or notch is increased with this approach. In addition, as the frequency of oscillations increase, mechanical limits attributable to inertia are soon reached with the manufacturing equipment employed.
Another approach at solving the problem has been that of making the units of twisted wire pairs into a continuous rather than a periodically reversing lay. This tends to alleviate the mechanical problem associated with quick reversals in faceplate oscillations. However, this approach prevents tandemization of multiple unit formation with that of stranding. Thus, when this process is used the units must be formed in batches to be subsequently stranded into cores.
It therefore is seen that an effective and economic solution for the problem just described yet remains to be had. It is the provision of a solution to this to which the present invention is directed.